Hemiepiphytic vine, stem scandent, green to gray-green, drying pale yellowish brown, unscented, leaf scars inconspicuous, 1.2 cm long, 8 mm wide, obscured by cataphylls; internodes smooth, semiglossy, 6--12 cm long, 1--2 cm diam., longer than broad, moderately green, drying khaki-colored, epidermis fissured transversely; roots drying light reddish brown, smooth, weakly glossy, 20--30 cm long, 2--3 mm diam., 4--6, at the nodes; cataphylls subcoriaceous, bluntly 2-ribbed or unribbed, pale green to cream-colored, promptly deciduous; petioles 12--24 cm long, 2--4 mm diam., subterete to broader than thick to broadly D-shaped, weakly spongy, bluntly flattened to broadly sulcate adaxially, rounded abaxially, with adaxial margins blunt, surface pale or dark green streaked and demarcated from blade by dark green ring around apex; blades broadly ovate, moderately coriaceous, abruptly acuminate at apex (the acumen tightly inrolled, 1--4 mm long), weakly subcordate at base, 12.5--30 cm long, 13--21.5 cm wide (0.7--1(1.7) times longer than wide), (0.65--2 times longer than petiole), about equal in length to petiole, broadest more or less near the middle, margins straight, upper surface dark green, semiglossy, somewhat pruinose, lower surface glossy to weakly glossy, paler; anterior lobe 12--20.5 cm long, 10.7--22 cm wide (1.85--3 times longer than posterior lobes); posterior lobes 4--8 cm long, 3.5--9.5 cm wide, broadly rounded to obtuse; sinus arcuate, sometimes parabolic, 3.4--6 cm deep; midrib broadly convex to weakly raised, concolorous to paler than surface above, weakly to broadly convex, paler than surface below; basal veins about 3 per side, obscurely and scarcely more conspicuous than primary laterals; posterior rib lacking; primary lateral veins (3)4--5 per side, obscure above; minor veins about as conspicuous as the primary laterals, arising from the midrib only. INFLORESCENCES spreading, as long as leaves, 1 per axil; peduncle 14--25 cm long, 2--6 mm diam., subterete, green; spathe 6.5--15 cm long, (0.3--1 times longer than peduncle); spathe blade green outside, pale green inside; spathe tube ca. 5 cm long, green outside, red or violet-purple inside; spadix sessile, 8 cm long, broadest below the middle; pistillate portion medium green to pale greenish yellow, more or less cylindrical, 1.9--2 cm long, 6.5--7 mm diam. at apex, 7--8 mm diam. at middle, 6.5--7 mm wide at base, with 13 (per cm) flowers per spiral; staminate portion 5.9--8.3 cm long; fertile staminate portion white, tapered toward apex, 5--9 mm diam. at base, 5--6 mm diam. at middle, 0.9 mm diam. ca. 1 cm from apex, broadest at the base, mostly narrower than the pistillate and sterile portions; sterile staminate portion broader than the pistillate portion, 6--9 mm diam.; pistils 1.2--1.4 mm long, 0.8--1.1 mm diam.; ovary 6--7-locular, 0.8--1 mm long, 0.8--1.1 mm diam., with sub-basal placentation, walls drying weakly warty; locules 0.8 mm long, 0.3 mm diam., ovule sac 0.7--0.8 mm long; ovules 1 per locule, contained within transparent ovule sac, 0.4 mm long, as long as funicle; funicle 0.4 mm long; style 0.2 mm long, 1 mm diam., similar to style type B; style apex flat; stigma unlobed, subdiscoid to somewhat cylindrical, 0.8--0.9 mm diam., 0.2 mm high, covering almost entire style apex, centered on stylar canal pores; the androecium truncate, margins irregularly to bluntly 4--6-sided, 1.2 mm long, 0.8 mm diam. at apex; thecae more or less cylindrical, 0.3 mm wide, more or less parallel to one another, contiguous; sterile staminate flowers acutely or bluntly and irregularly 4--6-sided, 1.2--2 mm long, 0.7--1.1 mm wide. INFRUCTESCENCE with fruits whitish. Seeds (dried) ca. 20 per locule, tan, 1--1.2 mm long, 0.5--0.6 mm diam., finely ridged with much finer cross-etching.

Flowering in Philodendron microstictum occurs during the dry season and early rainy season, with flowering collections known from January, April, and May, and post-anthesis collections from February, March, May, and July. Immature fruits have been collected in May.

Philodendron microstictum is endemic to Costa Rica (though to be expected on the Burica Península in adjacent Panama), where it is restricted to the Pacific slope, at 50 to 450 m elevation in wetter parts of Tropical moist forest, Tropical wet forest (T-wf), and Tropical wet forest basal belt transition life zones.

Philodendron microstictum is a member of P. sect. Calostigma subsect. Glossophyllum ser. Ovata. This species is characterized by its scandent habit, long, moderately slender stems drying pale yellowish brown, subterete petioles which are about as long as the blades, and broadly ovate, weakly subcordate blades (which may be as wide or wider than long) with an arcuate sinus and obscure primary lateral and basal veins (sometimes with only the basal veins visible). Also characteristic is the externally green spathe (red within the tube) with the peduncle as long or longer than the petiole.

Philodendron microstictum is not easily confused with any other species. It has been confused by some with P. scandens which differs in having more narrowly ovate blades with prominent major veins and a spathulate to parabolic sinus, and much more short-pedunculate inflorescences (with the peduncles much shorter than the petioles).

This species is perhaps related to P. chirripoense which is also a vine with inflorescences longer than leaves and has blades of similar color and texture. That species differs in having longer, more slender internodes which dry dark brown and by its narrowly ovate, subcordate blades which are more than twice as long as broad and have more prominent primary lateral veins.

Additional specimens examined.